Switching power supply circuits are widely used in coordination with various portable and automated electronic devices and instruments such as LCDs. Referring to FIG. 2, a typical switching power supply circuit 1 includes a power source 10, a pulse width modulation (PWM) circuit 11, a first switching circuit 12, a second switching circuit 13, a first transformer 15, a second transformer 16, and two filter circuits 14.
Each filter circuit 14 includes a resistor 141 and a series capacitor 142. The PWM circuit 11 includes a first output 111 and a second output 112. The first switching circuit 12 includes a first transistor 121 and a second transistor 122. Source electrodes of the first transistor 121 and the second transistor 122 are connected to ground. Gate electrodes of the first transistor 121 and the second transistor 122 are connected to the first output 111. The first output 111 is configured to output pulse voltages to switch on and switch off the first transistor 121 and the second transistor 122. The first and second transistors 121, 122 are metal-oxide-semiconductor field-effect transistors (MOSFETs).
The second switching circuit 13 includes a third transistor 131 and a fourth transistor 132. Source electrodes of the third transistor 131 and the fourth transistor 132 are connected to ground. Gate electrodes of the third transistor 131 and the fourth transistor 132 are connected to the second output 112. The second output 112 is configured to output pulse voltages to switch on and switch off the third transistor 131 and the fourth transistor 132. The third and fourth transistors 131, 132 are MOSFETs.
The first transformer 15 includes a first primary winding 151, a second primary winding 152, and a first secondary winding 154. The first primary winding 151 and the second primary winding 152 have a first common terminal 153. The first common terminal 153 is connected to the power source 10, and is further connected to ground via the filter circuit 14. The first primary winding 151 further includes a first terminal (not labeled). The second primary winding 152 further includes a second terminal (not labeled).
The second transformer 16 includes a third primary winding 161, a fourth primary winding 162, and a second secondary winding 164. The third primary winding 161 and the fourth primary winding 162 have a second common terminal 163. The second common terminal 163 is connected to the power source 10, and is further connected to ground via the filter circuit 14. The third primary winding 161 further includes a third terminal (not labeled). The fourth primary winding 162 further includes a fourth terminal (not labeled).
The first terminal of the first transformer 15 and the third terminal of the second transformer 16 are connected to ground via a first capacitor (not labeled), and are further connected to drain electrodes of the first transistor 121 and the second transistor 122. The second terminal of the first transformer 15 and the fourth terminal of the second transformer 16 are connected to ground via a second capacitor (not labeled), and are further connected to drain electrodes of the third transistor 131 and the fourth transistor 132.
When the switching power supply circuit 1 works, the first output 111 and the second output 112 provide pulse voltages to the first and second switching circuits 12, 13 respectively. When the pulse voltage provided by the first output 111 is a high voltage, the pulse voltage provided by the second output 112 is a low voltage. That is, when the first transistor 121 and the second transistor 122 of the first switching circuit 12 are switched on, the third transistor 131 and the fourth transistor 132 of the second switching circuit 13 are switched off, and vice versa. Because the two transistors 121 and 122 of the first switching circuit 12 are simultaneously switched on, the two transistors 121 and 122 may be damaged or even destroyed by heat generated thereat after the two transistors 121 and 122 have been working for a long time. The two transistors 131 and 132 of the second switching circuit 13 have the same problem.
Furthermore, when the first switching circuit 12 is switched on, and the second switching circuit 13 is switched off, the power source 10, the first primary winding 151 and the first switching circuit 12 form a closed loop, and the power source 10, the third primary winding 161 and the first switching circuit 12 also form a closed loop. The first primary winding 151 and the third primary winding 161 are connected to ground in parallel via the first switching circuit 12. Thus, the parallel first and second primary windings 151, 161 electromagnetically interfere with each other. Therefore, the outputs of the first and second secondary windings 154, 164 are liable to be non-uniform and unstable.
Similarly, when the second switching circuit 13 is switched on, and the first switching circuit 12 is switched off, the power source 10, the second primary winding 152 and the second switching circuit 13 form a closed loop, and the power source 10, the fourth primary winding 162 and the second switching circuit 13 also form a closed loop. The second primary winding 152 and the fourth primary winding 162 are connected to ground in parallel via the second switching circuit 13. Thus, the parallel second and fourth primary windings 151, 161 electromagnetically interfere with each other. Therefore, the outputs of the first and second secondary windings 154, 164 are liable to be non-uniform and unstable.
In summary, due to the above-described factors, the reliability and quality of the switching power supply circuit 1 may be unsatisfactory.
What is needed, therefore, is a switching power supply circuit that can overcome the above-described deficiencies.